The protection of metal surfaces against corrosion by the application thereto of coatings containing lead or chromium pigments has been described in the patent art and in technical publications and certain of such coatings have, to some extent, been employed industrially.
Moreover, chromic acid and its salts are well recognized to be effective corrosion inhibiting agents for application to metallic surfaces such as iron, steel, zinc, aluminum, copper and alloys of these. Thus, in an earlier patent of the present inventor, U.S. Pat. No. 3,063,877, there is described the protective treatment of metal surfaces, particularly such surfaces having thereon a previously applied conversion coating, by applying to said surface an aqueous solution containing chromium compounds, designated "mixed chromium compounds", including hexavalent and lower valent chromium compounds or complexes, obtained by partial reduction of hexavalent chromium.
In a later patent of the present inventor, U.S. Pat. No. 4,161,409 chromium compositions obtained by partial reduction of hexavalent chromium, are converted to pigment form by evaporating the aqueous reaction product of such reduction to dryness and pulverizing the dried product. Such pigments, with or without extenders, incorporated in conventional vehicles serve as corrosion inhibitors in metal coating compositions.
Another approach to the treatment of metal surfaces to improve corrosion resistance and to prepare such surfaces to receive a siccative finish, is disclosed in U.S. Pat. Nos. 3,185,596; 3,189,488 and 3,189,489 issued to the present inventor. As disclosed in these patents, coatings on metal surfaces having improved flexibility, good paint adhesion, and impact and corrosion resistance are obtained when the applied coating comprises an aqueous solution of mixed chromium compounds (such as that obtained by partial reduction of hexavalent chromium) to which there is added certain water soluble polymers. Among the polymers disclosed in these patents are included: water-soluble and water-dispersible polyacrylic acid compounds, water soluble polymers of hydroxyethyl ethers of cellulose and polyvinyl alcohols.
Recently issued U.S. Pat. No. 4,183,772 of J. W. Davis, assigned to Union Carbide Corporation, refers to said U.S. Pat. No. 3,185,596 of the present inventor as one of the most satisfactory of the "no waste" pretreatments for application to metals for improving corrosion resistance and preparation of the metal surface to receive a siccative finish. The Davis patent describes a modification of the cited U.S. Pat. No. 3,185,596, and involves the use of solutions comprising hexavalent and trivalent chromium in association with polyacrylic acid, phosphoric acid and water dispersible acrylic emulsion polymer. This mixture is stated by patentee to extend the utility of pretreating compositions of the cited '596 patent for application to an extended variety of paint systems and paint types.